EAE is an inflammatory disease of the central nervous system initiated by myelin antigen-specific encephalitogenic CD4+ Th1 cells. After encountering antigen, T cells receive signal 1 through the TCR and signal 2 through "positive" costimulatory molecules leading to full activation. Other costimulatory molecules such as CTLA4 provide a negative signal for T cell activation and may be important for terminating immune responses. Recently, another pathway that provides negative signaling to T cells has been described, the PD1- PDL1/2 pathway. The main goal of this proposal is to define the functions and mechanisms of the PD1 pathway in regulating EAE. We have unique reagents (monoclonal antibodies and fusion proteins) and animal models (gene knockout and TCR transgenic animals) that will enable us to dissect the role of this pathway in a clinically relevant disease model. We will use these tools to study the following: Aim 1: What is the function of PD1-PDL pathway in regulating EAE in vivo? Our hypothesis is that the PD1-PDL1/PDL2 pathway negatively regulates autoimmune responses in vivo. Using blocking monoclonal antibodies as tools to investigate the functions and of this new pathway in autoimmune responses in acute and chronic EAE, and in relapsing disease. We will define the role of this pathway in regulating autoreactive T cells in vivo. We will also study the functions of this new pathway in active disease and passively induced disease. Aim 2: Role of PD 1- PDL1 in tolerance to autoantigens. Our hypothesis is that signaling through PD1 promotes tolerance in autoreactive T cells. We will use a signaling fusion protein targeting PDL1 to induce tolerance in EAE. We will use unique in vitro and in vivo assays including MOG TCR transgenic animals to better understand the mechanisms of targeting the PD1 costimulatory pathway in vivo. PDL1 and PDL2, are expressed on both professional bone marrow derived antigen-presenting cells and on parenchymal cells, by using PD1, PDL1, and PDL2 deficient mice we will investigate the role of parenchymal versus lymphoid cell expression of the ligands on the function of this pathway in regulating autoimmune responses. Aim 3: Interactions between PD1-PDL1/PDL2 pathway and other "positive" and "negative" regulatory pathways. Our hypothesis is that PD1-PDL pathway plays an important role in regulating EAE particularly in the absence of CD28. We will define the interactions between the PD1 pathway and other CD28 homologues (CD28, CTLA4, ICOS) in regulating EAE.